Artificial limbs have come a long way in recent years with the development of prostheses that can be controlled directly by the patient’s nerves. The problem is, links between living nerves and the prostheses break down over time, which makes permanent attachment and practical control difficult. To understand why this happens and to help give patients more control over their prostheses, DARPA has instituted a number of programs aimed at improving neural interfaces and allowing amputees to have better control of advanced prostheses in the near term.

According to the US Defense Department, over 2,000 US military personnel have suffered major amputations since the year 2000. With the US government committed to providing these amputees with advanced prosthetic technology, DARPA’s Reliable Neural-Interface Technology (RE-NET) program studies why neural interfaces stop working properly and why a prosthetic limb can’t understand nerve signals well enough to move with high speed and resolution.

RE-NET is divided into a number of teams focusing on a number of different approaches. One is the Histology for Interface Stability over Time team that studies interactions between biotic (living) and abiotic (artificial) systems and why they fail, as well as other factors involved.

The others are the Reliable Peripheral Interfaces and the Reliable Central-Nervous-System Interfaces teams. They are working to build systems to study how interfaces connected directly to the brain and spinal cord can be used to control prostheses with high reliability and performance.

An example of this was demonstrated at the Rehabilitation Institute of Chicago where Former Army Staff Sgt. Glen Lehman, injured in Iraq, used neural interfaces and existing muscles to demonstrate simultaneous joint control of a prosthetic arm. Lehman's arm relied on Targeted Muscle Re-innervation, which involves transferring multiple nerves to regions of a targeted muscle and then using independent signals from the nerves to control the prosthesis.

“Although the current generation of brain, or cortical, interfaces have been used to control many degrees of freedom in an advanced prosthesis, researchers are still working on improving their long-term viability and performance,” said Jack Judy, DARPA program manager. “The novel peripheral interfaces developed under RE-NET are approaching the level of control demonstrated by cortical interfaces and have better biotic and abiotic performance and reliability. Because implanting them is a lower risk and less invasive procedure, peripheral interfaces offer greater potential than penetrating cortical electrodes for near-term treatment of amputees. RE-NET program advances are already being made available to injured warfighters in clinical settings.”

One factor in controlling advanced prosthetic limbs is sensory feedback, Without it, an artificial arm is just a dead rod of metal and plastic that needs to be controlled by sight. Researchers at Case Western Reserve University used a Flat Interface Nerve Electrode (FINE) to demonstrate direct sensory feedback. Instead of using the patient’s residual nerves to just control the prosthesis, the researchers used the link to provide a rudimentary sense of touch. This way, the patient had some awareness of the limb and what it was doing, so it could be used to rummage in a bag or pick up objects hidden behind a curtain.